diff --git a/specs/core/1_beacon-chain.md b/specs/core/1_beacon-chain.md
index e7f53865d..74ad4f5f9 100644
--- a/specs/core/1_beacon-chain.md
+++ b/specs/core/1_beacon-chain.md
@@ -253,11 +253,11 @@ class BeaconState(phase0.BeaconState):
     current_justified_checkpoint: Checkpoint
     finalized_checkpoint: Checkpoint
     # Phase 1
-    shard_states: Vector[ShardState, MAX_SHARDS]
+    shard_states: List[ShardState, MAX_SHARDS]
     online_countdown: Bytes[VALIDATOR_REGISTRY_LIMIT]
     current_light_committee: CompactCommittee
     next_light_committee: CompactCommittee
-    
+    # Custody game
     # TODO: custody game refactor, no challenge-records, immediate processing.
     custody_challenge_index: uint64
     # Future derived secrets already exposed; contains the indices of the exposed validator
@@ -607,7 +607,7 @@ def process_attestations(state: BeaconState, block_body: BeaconBlockBody, attest
 ```python
 def verify_shard_transition_false_positives(state: BeaconState, block_body: BeaconBlockBody) -> None:
     # Verify that a `shard_transition` in a block is empty if an attestation was not processed for it
-    for shard in range(MAX_SHARDS):
+    for shard in range(ACTIVE_SHARDS):
         if state.shard_states[shard].slot != state.slot - 1:
             assert block_body.shard_transition[shard] == ShardTransition()
 ```
diff --git a/specs/core/1_phase1_fork.md b/specs/core/1_phase1_fork.md
new file mode 100644
index 000000000..f5ab658f5
--- /dev/null
+++ b/specs/core/1_phase1_fork.md
@@ -0,0 +1,107 @@
+# Ethereum 2.0 Phase 1 -- From Phase 0 to Phase 1
+
+**Notice**: This document is a work-in-progress for researchers and implementers.
+
+## Table of contents
+
+<!-- TOC -->
+
+ TODO
+
+<!-- /TOC -->
+
+## Introduction
+
+This document describes the process of moving from Phase 0 to Phase 1 of Ethereum 2.0.
+
+## Configuration
+
+TODO: very unstable/experimental. PLACEHOLDER.
+
+| Name | Value | Unit |
+| - | - | - |
+| `PHASE_1_FORK_VERSION` | `0x00000001` | `Version` |
+| `INITIAL_GASPRICE` | `10` | `Gwei` |
+
+## Fork to Phase 1
+
+### Fork trigger.
+
+TBD. Social consensus, along with state conditions such as epoch boundary, finality, deposits, active validator count, etc. may be part of the decision process to trigger the fork.
+
+### Upgrading the state
+
+After `process_slots` of Phase 0 finishes, but before the first Phase 1 block is processed, an irregular state change is made to upgrade to Phase 1.
+
+```python
+def upgrade_to_phase1(pre: phase0.BeaconState) -> BeaconState:
+    post = BeaconState(
+        genesis_time=pre.genesis_time,
+        slot=pre.slot,
+        fork=Fork(
+            previous_version=pre.current_version,
+            current_version=PHASE_1_FORK_VERSION,
+            epoch=get_current_epoch(pre),
+        ),
+        # History
+        latest_block_header=pre.latest_block_header,
+        block_roots=pre.block_roots,
+        state_roots=pre.state_roots,
+        historical_roots=pre.historical_roots,
+        # Eth1
+        eth1_data=pre.eth1_data,
+        eth1_data_votes=pre.eth1_data_votes,
+        eth1_deposit_index=pre.eth1_deposit_index,
+        # Registry
+        validators=List[Validator, VALIDATOR_REGISTRY_LIMIT](
+            Validator(
+                pubkey=phase0_validator.pubkey,
+                withdrawal_credentials=phase0_validator.withdrawal_credentials,
+                effective_balance=phase0_validator.effective_balance,
+                slashed=phase0.slashed,
+                activation_eligibility_epoch=phase0_validator.activation_eligibility_epoch,
+                activation_epoch=phase0_validator.activation_eligibility_epoch,
+                exit_epoch=phase0_validator.exit_epoch,
+                withdrawable_epoch=phase0_validator.withdrawable_epoch,
+                next_custody_secret_to_reveal=,
+                max_reveal_lateness=,
+            ) for phase0_validator in pre.validators
+        ),
+        balances=pre.balances,
+        # Randomness
+        randao_mixes=pre.randao_mixes,
+        # Slashings
+        slashings=pre.slashings,
+        # Attestations
+        # previous_epoch_attestations is cleared on upgrade. 
+        previous_epoch_attestations=List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH](),
+        # empty in pre state, since the upgrade is performed just after an epoch boundary.
+        current_epoch_attestations=List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH](),
+        # Finality
+        justification_bits=pre.justification_bits,
+        previous_justified_checkpoint=pre.previous_justified_checkpoint,
+        current_justified_checkpoint=pre.current_justified_checkpoint,
+        finalized_checkpoint=pre.finalized_checkpoint,
+        # Phase 1
+        shard_states=List[ShardState, MAX_SHARDS](
+            ShardState(
+                slot=0,
+                gasprice=INITIAL_GASPRICE,
+                data=Root(),
+                latest_block_root=Hash(),
+            ) for i in range(ACTIVE_SHARDS)
+        ),
+        online_countdown=Bytes[VALIDATOR_REGISTRY_LIMIT](
+            ONLINE_PERIOD for i in range(len(pre.validators))            
+        ),
+        current_light_committee=CompactCommittee(),  # computed after state creation
+        next_light_committee=CompactCommittee(),
+        # Custody game
+        custody_challenge_index=0,
+        exposed_derived_secrets=Vector[List[ValidatorIndex, PLACEHOLDER],
+                                        EARLY_DERIVED_SECRET_PENALTY_MAX_FUTURE_EPOCHS]()
+    )
+    post.current_light_committee = get_light_client_committee(post, post.epoch)
+    post.next_light_committee = get_light_client_committee(post, post.epoch + 1)
+    return post
+```